Phase-locked loop demodulators contribute significantly to performance improvements in many types of receiving systems. Particular improvements are conspicuous in FM stereophonic receivers. One of the important performance characteristics of such a receiver is the ability to reduce the FM detection threshold, thereby improving the signal-to-noise ratio on weak signals. This ability is substantially derived from the inherent low pass frequency response characteristic of phase-locked loop demodulators. However, unless this frequency response characteristic is optimized, degradation of the separation between the distinct stereophonic signal components is introduced. Clearly this degradation cannot be eliminated by subsequent receiver stages and will ultimately appear at the receiver output. Known prior art phase-locked loop demodulators do not include circuitry for optimizing stereophonic separation.
Many prior art receivers incorporate noise-activated muting circuits to interrupt the receiver output during the absence of input signals of at least a minimum desired amplitude. These circuits are typically activated by a band-limited or a high pass filtered noise signal outside the spectrum of the desired demodulated signal. Typical prior art noise muting circuits required at least a high-frequency bandpass filter and noise amplifier. Because phase-locked loop demodulators possess a relatively narrow noise bandwidth and provide a reduced high-frequency noise output, additional noise amplification is required for operation with muting circuits.